Essentially, the present invention is an improvement over an established alloy disclosed in U.S. Pat. No. 3,859,060. This patent encompasses a commercial alloy known as alloy 617, a product which has been produced and marketed for a number of years. Nominally, the 617 alloy contains about 22% chromium, 9% molybdenum, 1.2% aluminum, 0.3% titanium, 2% iron, 12.5% cobalt, 0.07% carbon, as well as other constituents, including 0.5% silicon, one or more of boron, manganese, magnesium, etc., the balance being nickel. The virtues of alloy 617 include (i) good scaling resistance in oxidizing environments, including cyclic oxidation, at elevated temperature, (ii) excellent stress rupture strength, (iii) good tensile strength and ductility at both ambient and elevated temperatures, etc.
Alloy 617 also possesses structural stability under, retrospectively speaking, what might be characterized as, comparatively speaking, moderate service conditions. But as it has turned out it is this characteristic which has given rise to a problem encountered commercially for certain intended and desired applications, e.g., high temperature gas feeder reactors (HTGR). This is to say, when the alloy was exposed to more stringent operating parameters of temperature (1800.degree. F.) and time (1000-3000+ hours) an undesirable degradation in structural stability occurred, though stress rupture, tensile and oxidation characteristics remained satisfactory.
Apparently, what happened was that prior to the 1800.degree. F./1000+ hour operating conditions, the test temperature for stability study was usually not higher than 1600.degree. F. And if higher temperatures were considered, short term exposure periods, circa 100 hours, were used. Longer term periods (circa 10,000 hours or more) were used but at the lower temperatures, i.e., not more than 1300.degree. F.-1400.degree. F.
Apart from temperature/time operating conditions, the problem would not surface because in many applications structural stability was not critically important, e.g., boats used for catalyst-grid supports, heat treating baskets, reduction boats used in refining certain metals, etc.
Accordingly, the problem became one of ascertaining the cause(s) for the stability deterioration at upwards of 1800.degree. F.-2000.degree. F. for periods well exceeding 1000 hours, and evolving, if possible, a new alloy which would result in enhanced stability under such operating conditions but without incurring a detrimental sacrifice in stress-rupture/oxidation/tensile properties.